Method for depositing metal oxide film in liquid environment

ABSTRACT

A method for depositing a metal oxide film in a liquid environment is provided, and includes steps of: dissolving an oxidizing agent in solvent with hydrogen bond to form a solution, and placing a substrate into the solution for performing a deposition reaction to deposit a metal oxide hydroxide film on the substrate. The oxidizing agent is potassium permanganate, potassium chromate, or potassium dichromate, a reaction temperature of the deposition reaction ranges from 1 to 99 degrees Celsius, and a reaction pressure environment of the deposition reaction is an atmospheric pressure environment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Taiwan Patent Application No.107147031, filed on Dec. 25, 2018, and is partly disclosed in a thesisentitled “Redox-assisted multicomponent deposition of ultrathinamorphous metal oxides on arbitrary substrates: highly durable cobaltmanganese oxyhydroxide for efficient oxygen evolution” on Oct. 7, 2018completed by Ren-Huai Jhang, Chang-Ying Yang, Ming-Chi Shih, Jing-QianHo, Ya-Ting Tsai, and Chun-Hu Chen, and thus the disclosure of which isincorporated herein by reference.

FIELD OF INVENTION

The present disclosure relates to a method of producing a metal oxidefilm, and specifically to a method for depositing the metal oxide filmby electroless plating in a liquid environment.

BACKGROUND OF INVENTION

Ultrathin multicomponent deposition (<10 nm) over large dimensions is ofgreat interest to engineers and scientists, but it commonly suffers fromisland-like discontinuity and elemental segregation. Transition metaloxide thin films with uniform thickness and continuous coverage areshown to be essential in a wide range of modern devices andarchitectures, including flexible and wearable electronics.

Well-established chemical and physical depositions (e.g. chemical vapordeposition, evaporation, sputtering, atomic layer deposition, etc.)require a high standard of operation conditions (e.g. delicatechemicals, high vacuum/energy consumption, expensive instrumentation,etc.) but provide limited production scales. Solution processabledeposition, due to its low-cost and easy operation, emerges to explorelow temperature, massive-scale fabrication on substrates of lowthermal-durability (plastics/soft materials) and complex 3D structures.

Many typical solution-processable depositions (e.g., drop-casting,sol-gel, spray/dip/spin coating, etc.) require pyrolysis to remove anorganic residue and to promote film adhesion, however, they are notsuitable for amorphous/metastable deposition and soft/flexiblesubstrates. Electrodeposition may be considered as a substitute to avoidpyrolysis, but highly conductive substrates are generally needed. Thedrawbacks of pinhole formation, rapid deposition rates hinderingultrathin coatings, and inhomogeneous multi-element deposition due tovaried deposition potentials for individual elements, also limit itscontrol in active site formation and charge transport resistance forelectrocatalysis.

Thin films of earth-abundant transition metal oxides with easydeposition are promising candidates to achieve efficient oxygenevolution reaction (OER) at a reasonable cost. Notably, studies haveshown that amorphous transition metal oxides, including intermediatestates present during electrocatalysis, possess greater activities thantheir crystalline forms.

However, since a pyrolysis step is commonly involved in solutionprocessable deposition, that is not suitable for a soft plasticsubstrate, and an amorphous product cannot be obtained. Only a fewexamples of amorphous oxide coatings have been successfully reported,but their high resistivity also causes difficulties in electrocatalysis.

SUMMARY OF INVENTION

An object of the present disclosure is to provide a method fordepositing a metal oxide film in a liquid environment and the method isimplemented by depositing a multi-component metal oxide film ondifferent substrates in the liquid environment in order to meet massproduction requirements.

In order to achieve the above object, the present disclosure providesthe method for depositing the metal oxide film in the liquidenvironment, including steps of: (S1) dissolving an oxidizing agent in asolvent with hydrogen bonds to form a solution; and (S2) placing asubstrate into the solution for performing a deposition reaction todeposit a metal oxide hydroxide film on the substrate; wherein theoxidizing agent is potassium permanganate, potassium chromate, orpotassium dichromate, a reaction temperature of the deposition reactionranges from 1 to 99 degrees Celsius, and a reaction pressure environmentof the deposition reaction is an atmospheric pressure environment.

In an embodiment of the present disclosure, in the step (S1), furtherincludes a step of mixing a reducing agent and the oxidizing agent basedon a molar ratio of the reducing agent to the oxidizing agent, in orderto dissolve the oxidizing agent and the oxidizing agent in the solventwith hydrogen bonds to form the solution.

In an embodiment of the present disclosure, the reducing agent isselected from the group consisting of a divalent cobalt compound, adivalent iron compound, a divalent nickel compound, a divalent manganesecompound, and a first transition metal ionic compound.

In an embodiment of the present disclosure, the molar ratio of thereducing agent to the oxidizing agent ranges from 9:1 to 1:3.

In an embodiment of the present disclosure, in the step (S1), furtherincludes a step of adding an additive containing an anion into thesolution, wherein the anion of the additive is selected from metal saltions.

In an embodiment of the present disclosure, further including a step(S3) after the step (S2), wherein the step (S3) includes: causing themetal oxide hydroxide film to be calcined by a calcination process in acalcination temperature range and under a gas environment to produce acalcined metal oxide film, wherein the calcination temperature rangesfrom 250 to 800 degrees Celsius.

In an embodiment of the present disclosure, the gas in the gasenvironment is air in an atmospheric environment.

In an embodiment of the present disclosure, the gas in the gasenvironment is argon, nitrogen, or oxygen.

In an embodiment of the present disclosure, a duration of thecalcination process ranges from 1 to 12 hours.

In an embodiment of the present disclosure, the substrate is selectedfrom a group consisting of silicon crystal, carbohydrate, glass, nickelfoam, metal, metal oxide, organic matter, organic polymer, carbonmaterial, and glassy carbon electrode.

In an embodiment of the present disclosure, the solvent with hydrogenbonds is deionized water with an impedance of 18.2 MΩ·cm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart showing a method of depositing a metal oxide filmin a liquid environment according to an embodiment of the presentdisclosure.

FIGS. 2-10 are schematic diagrams (1) to (9) that illustrate adeposition procedure and characterization of CMOH_(acetate); wherein

FIG. 2 is a diagram of dipping a transparent FTO substrate into theaqueous reaction mixture of Co(OAc)₂ and KMnO₄;

FIG. 3 shows a diagram of aging FTO for 15 minutes (min.);

FIG. 4 shows a diagram of removing the FTO substrate after completedeposition (the darker contrast area), processes from FIGS. 2 to 4 areperformed in sequence;

FIG. 5 shows a diagram of an SEM image;

FIG. 6 shows a diagram of AFM data of the coatings;

FIG. 7 shows a diagram of grazing-angle XRD patterns of thin and thickCMOH deposition by Co(OAc)₂ and CoSO₄;

FIG. 8 shows a diagram of an HR-TEM image of CMOH_(acetate) foils;

FIG. 9 shows a diagram of CMO_(acetate) after annealed at 500° C.; and

FIG. 10 shows a diagram of a TEM image of CMOH after 10000 cycles of OERtests;

wherein insets in FIGS. 8-10 are corresponding Fourier transformpatterns of the CMOH area.

FIGS. 11-18 are schematic diagrams (1) to (8) that illustrate depositioncondition tests; wherein

FIG. 11 shows a diagram of photographs of parallel deposition on sixindividual FTO substrates;

FIG. 12 shows a diagram of products of six coated FTO films from FIG. 11with uniform coating contrasts;

FIG. 13 shows a diagram of Large scale deposition of CMOH with a highlyordered array over a 100 cm² FTO film;

FIG. 14 shows a diagram of CMOH deposition at room temperature (R.T.)and others, wherein arrows indicate the corners of the deposition areas;

FIG. 15 shows a diagram of transparency of CMOH coatings shown in FIG.14;

FIG. 16 shows a diagram of CMOH adhesion tests by Scotch tape peelingfor more than 100 cycles;

FIG. 17 shows a diagram of a cross-section profile of CMOH coating on aSi trench; and

FIGS. 18a-18f show diagrams of tests of arbitrary substrate deposition,wherein FIG. 18a shows a diagram of CMOH coated on 3D porous Ni foamwith the comparison of bare Ni foam, FIG. 18b shows a diagram ofhomogeneous deposition on the cylinder-shape surface of screw pairs,FIG. 18c shows a diagram of CMOH coating with transfer of large-sizecomplicated patterns onto a 100 cm² glass substrate by masking, FIG. 18dshows a diagram of the CMOH coated PET, where no appreciable crackingcan be observed after bending and folding for 100 cycles, FIG. 18e showsa diagram of the CMOH coated curved wooden surface, and FIG. 18f shows adiagram of CMOH coated on the metallic surface of Cu foils.

FIGS. 19-24 are schematic diagrams (1) to (6) that illustratespectroscopic and profile studies of CMOH; wherein

FIG. 19 shows a diagram of XPS data of Co 2p for uncalcined coating;

FIG. 20 shows a diagram of XPS data of Mn 2p for uncalcined coating;

FIG. 21 shows a diagram of XPS data of O 1s for uncalcined coating;

FIG. 22 shows a diagram of XAS spectra of Co K-edge;

FIG. 23 shows a diagram of XAS spectra of Mn K-edge; and

FIG. 24 shows a diagram of film composition profile.

FIGS. 25-29 are schematic diagrams (1) to (5) that illustrate QCMstudies of film growth with different precursor recipes, wherein alltests were conducted under identical preparation conditions; wherein

FIG. 25 shows a diagram of a comparison of cobalt and manganeseprecursor-only cases with the two precursor mixed condition;

FIG. 26 shows a diagram of a comparison of different anions in thecobalt precursors;

FIG. 27 shows a diagram of the effect of additional ligands on CMOHgrowth;

FIG. 28 shows a diagram of a cross-sectional SEM image and EDXS mappingof CMOH prepared by CoSO₄; and

FIG. 29 shows a diagram of CVs of cobalt precursors with differentanions and ligands.

FIGS. 30-33 are schematic diagrams (1) to (4) that illustrate MDsimulated studies and analysis of CMOH growth; wherein

FIG. 30 shows diagrams (a) to (f) illustrating MD simulated studies ofCMOH growth, wherein (a) and (b) are illustrations of simulation cellsof the sulfate system and acetate system while the simulation is inprogress after linking, removal of all the unreacted Co²⁺ and MnO₄ ⁻ions, counter ions (OAc⁻, SO₄ ²⁻), and solvent (H₂O) was conducted toenable the clear presentation of (MnO₄)—Co complexes; (c) is an enlargedview of a (MnO₄)—Co complex as a colloidal precipitate in cells (a) and(b); (d) is an enlarged view of surface-linked (MnO₄)—Co complexes (CMOHfilm) deposited on the SnO₂ substrate in (e) the sulfate system and (f)the acetate system;

FIG. 31 shows a diagram of numbers of Co—O—Mn bonds of the colloidalcomplexes, analyzed from cells of (a) and (b) in FIG. 30;

FIG. 32 shows a diagram of numbers of Co—O—Mn bonds of the surfacelinked complexes, analyzed from (e) and (f) in FIG. 30; and

FIG. 33 shows a diagram of RDF analysis of Co²⁺ ions with O atoms ofacetate (O_(acetate)) and sulfate (O_(sulfate)) ions.

FIGS. 34-39 are schematic diagrams (1) to (6) that illustrateelectrocatalytic oxygen evolution studies of CMOH coatings on FTO;wherein

FIG. 34 shows a diagram of a comparison of amorphous CMOH and calcinedCMO with benchmark RuO₂ recorded at 0.1 M KOH and an inset in FIG. 34 isa zoom-in plot;

FIG. 35 shows a diagram of the Tafel plot comparison of materials inFIG. 34;

FIG. 36 shows a diagram of current density-potential curves of CMOHcoatings produced with varied Co-to-Mn ratios in the reaction mixtures;

FIG. 37 is a stability comparison between amorphous and crystallinecoatings in i-t curves;

FIG. 38 is a stability comparison between amorphous and crystallinecoatings in LSV cycle tests; and

FIG. 39 is a comparison of coatings with different cobalt precursors.

FIGS. 40-43 are schematic diagrams (1) to (4) that illustrate currentdensity-potential curve comparisons of amorphous CMOH coated on typicalsubstrates for the OER; wherein

FIG. 40 shows a diagram of a schematic diagram of a currentdensity-potential curve comparison of amorphous CMOH coated on Ni foam;

FIG. 41 shows a diagram of a schematic diagram of a currentdensity-potential curve comparison of amorphous CMOH coated on Cu foils;

FIG. 42 shows a diagram of a schematic diagram of a currentdensity-potential curve comparison of amorphous CMOH coated on carboncloth; and

FIG. 43 shows a diagram of a schematic diagram of a currentdensity-potential curve comparison of amorphous CMOH coated on glassycarbon electrode (GCE).

FIGS. 44-47 are schematic diagrams (1) to (4) that illustrate the EDXSresults of CMOH_(acetate); wherein

FIG. 44 shows a diagram of SEM images of CMOH with the selected areahighlighted by red dashed lines for mapping analysis;

FIG. 45 shows a diagram of the EDXS results show the signals of Co andMn with the atomic ratios of Co/Mn=3.08; and

FIG. 46 and FIG. 47 show diagrams of mapping results of Co and Mndistribution corresponding to a dashed line area in FIG. 44.

FIG. 48 shows a schematic diagram that illustrates the Raman spectra ofCMOH show a broad band at 599 cm⁻¹, indicating the presence of amorphouscobalt oxide, and the Raman signals of crystalline Co₃O₄ are shown forcomparison, according to an embodiment of the present disclosure.

FIG. 49 shows a schematic diagram that illustrates the GIXRD pattern ofthe CMOH_(sulfate) after a calcination at 500° C., showing phasescorresponding to Co₃O₄, according to an embodiment of the presentdisclosure.

FIG. 50 shows a schematic diagram that illustrates a characterization ofCMOH_(acetate) cross-section under HR-TEM (also see FIG. 8). The labelof I in the film area shows the EDXS signals with the majority of Co andMn. The upper left inset shows the FFT patterns of I corresponding to anamorphous characteristic. The area labeled by II of FTO exhibits thestrong Sn signal. The corresponding high resolution TEM images (thelower right inset) show a lattice corresponding to (110) of FTO. Gasignal is due to the ion-beam of Ga in FIB, according to an embodimentof the present disclosure.

FIG. 51 shows a schematic diagram that illustrates AFM results ofCMOH_(acetate) show the film thickness around 11 nm prepared at 80° C.for 60 minutes.

FIGS. 52-54 are schematic diagrams that illustrate the XPS data of thecrystalline CMO_(acetate) after annealing at 500° C.; wherein

FIG. 52 shows a diagram of the XPS data of Co 2p;

FIG. 53 shows a diagram of the XPS data of Mn 2p; and

FIG. 54 shows a diagram of the XPS data of O 1s.

FIGS. 55-56 are schematic diagrams that illustrate photographs ofCo(OAc)₂-only and KMnO₄-only deposition on nonmental substrates of wood(see FIG. 55) and PET (see FIG. 56), wherein arrows in FIG. 55 indicatethe boundary between deposition and deposition-free areas forcomparison, and these results show no film formation by Co(OAc)₂-onlydeposition, while thin coating can be observed by KMnO₄-only deposition.

FIG. 57 shows a schematic diagram that illustrates a Faradaic efficiencytest of CMOH_(acetate) films. After four hours oxygen evolution, thefilms exhibit the Faradaic efficiency of nearly 100%.

FIG. 58 shows a schematic diagram that illustrates the Tafel plots ofCMOH_(acetate) samples prepared by different Co/Mn precursor ratios at80° C. for 15 minutes, wherein the Tafel slopes are summarized in thefollowing table.

FIG. 59 shows a schematic diagram that illustrates cobalt XPS data ofthe CMOH_(7/1) (Co/Mn precursor ratio of 7/1), showing the highsimilarity to CMOH_(3/1) (Co/Mn precursor ratio of 3/1), indicating thatCo³⁺ is still the main species of the CMOH film.

FIG. 60 shows a schematic diagram that illustrates UV-vis spectra ofCMOH_(acetate) deposited under varied deposition time at 80° C.

FIG. 61 shows a schematic diagram that illustrates a correspondingcalibration curve of FIG. 60 with 550 nm absorbance.

FIGS. 62-65 are schematic diagrams that illustrate characterization ofamorphous iron manganese oxide and ternary iron cobalt manganese oxidecoatings on SiO₂/Si wafers, wherein the SEM results reveal that boththese two coatings (iron manganese oxides in FIG. 62 and iron cobaltmanganese oxide in FIG. 63) are highly smooth and crack-free, their EDXSresults are respectively shown in FIG. 64 and FIG. 65 giving thecorresponding compositions of Fe:Mn=2.39:1 and Fe:Co:Mn=1:2.11:0.77, andinsets in FIG. 62 and FIG. 63 shows the photographs of the filmappearance on FTO.

FIGS. 66a, 66b, and 66c are photographs showing samples resulting of ametal oxide calcined film at 400, 600, and 800 degrees Celsius,according to an embodiment of the present disclosure, respectively.

FIGS. 67a and 67b are photographs showing samples resulting of a sampleof a substrate being different organic polymers, according to anembodiment of the present disclosure.

FIGS. 68a to 68c are photographs showing samples resulting of a sampleof a substrate being different organic matters, according to anembodiment of the present disclosure.

FIGS. 69a to 69c are photographs showing samples resulting of a sampleof a substrate being different carbon materials, according to anembodiment of the present disclosure.

FIG. 70 is a photograph showing a sample resulting of a substrate beinga carbohydrate, according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following description of the various embodiments is provided toillustrate the specific embodiments of the present disclosure.Furthermore, directional terms mentioned in the present disclosure, suchas upper, lower, top, bottom, front, rear, left, right, inner, outer,side, surrounding, central, horizontal, lateral, vertical, longitudinal,axial, radial, uppermost or lowermost, etc., which only refer to thedirection of drawings. Therefore, the directional terms used as aboveare for the purpose of illustration and understanding of the presentdisclosure, and are not intended to limit the present disclosure.

Please refer to FIG. 1, a method for depositing a metal oxide film in aliquid environment according to an embodiment of the present disclosurebelongs to a metal oxide film production method, and may includefollowing steps: (S1) dissolving an oxidizing agent in a solvent withhydrogen bonds to form a solution; and (S2) placing a substrate into thesolution for performing a deposition reaction to deposit a metal oxidehydroxide film on the substrate; wherein the oxidizing agent ispotassium permanganate (KMnO₄), potassium chromate (K₂CrO₄), orpotassium dichromate (K₂Cr₂O₇), a reaction temperature of the depositionreaction ranges from 1 to 99 degrees Celsius (° C.), and a reactionpressure environment of the deposition reaction is an atmosphericpressure environment. The following examples illustrate some embodimentsof the above method of the present disclosure, but are not limited asdescribed here.

For example, the solvent with hydrogen bonds may be selected from wateror the like, such as alcohol. It is understood that, water and alcoholboth have hydrogen bonds and are mutually soluble, and the boilingpoints of water and alcohol are slightly different (such as water isabout 100° C. and alcohol is about 78.4° C.). The choice of otheravailable solvents is well understood by those of ordinary skill in theart and will not be described here. In the following, only water istaken as an example to illustrate the implementation in an aqueousenvironment. For example, the water may also be selected from deionized(DI) water with an impedance of 18.2 MΩ·cm to promote the reactionquality, but that is not limited as described here.

In an embodiment of the present disclosure, in the step (S1), anadditive containing an anion is added into the solution, wherein theanion of the additive is selected from metal salt ions, such as themetal including cobalt (Co), nickel (Ni), iron (Fe), manganese (Mn),vanadium (V), titanium (Ti), chromium (Cr), copper (Cu), and zinc (Zn).The salt may be a nitrate salt, a sulfate salt, an acetate salt, ahalogen salt or the like of the above metal. Specifically, the anion ofthe additive may be selected to be any anion as below, such as acetate,sulfate, sulfite, nitrate, halogen anion, thiosulfate, hydrogen sulfate,sulfite, hydrogen sulfite, persulfate, arsenate, arsenate, borate,bicarbonate, carbonate, hydroxide, perchlorate, chlorite, hypochlorite,chlorate, nitrite, acetylacetonate or ethylenediaminetetraacetic acid,that is but not limited as described here. Thus, differentconcentrations of the solution, and different film growth times (such as5 minutes to more than 24 hours) can be used to control the filmthickness (<10 nm), so as to control different film thicknesses andgrowth rates by producers.

In an embodiment of the present disclosure, in the step (S1), a reducingagent and the oxidizing agent are mixed based on a molar ratio of thereducing agent to the oxidizing agent, in order to dissolve theoxidizing agent and the oxidizing agent in the solvent (such as water)with hydrogen bonds to form the solution. For example, the reducingagent may be selected from the group consisting of a divalent cobaltcompound (Co²⁺), a divalent iron compound (Fe²⁺), a divalent nickelcompound (Ni²⁺), a divalent manganese compound (Mn²⁺), and a firsttransition metal ionic compound. In addition, the molar ratio of thereducing agent to the oxidizing agent may be ranged from 9:1 to 1:3.

For example, the divalent cobalt compound may be selected from cobaltacetate (Co(CH₃COO)₂), cobalt sulfate (CoSO₄), cobalt nitrate(Co(NO₃)₂), cobalt chloride (CoCl₂) or acetoacetate cobalt.(C₁₅H₂₁CoO₆); the divalent iron compound may be selected from ferrousacetate (Fe(CH₃COO)₂), ferrous sulfate (FeSO₄) or ferrous nitrate(Fe(NO₃)₂); the divalent nickel compound may be selected from sulfuricacid Nickel (NiSO₄), nickel nitrate (Ni(NO₃)₂) or nickel chloride(NiCl₂); the divalent manganese compound may be selected from manganeseacetate (Mn(CH₃COO)₂) or manganese sulfate (MnSO₄); and the firsttransition metal ionic compound may be selected from a compound ofvanadium, titanium, chromium, copper, or zinc ions, but that is notlimited as described here.

In an embodiment of the present disclosure, a reaction time of thedeposition reaction may be ranged from 5 minutes to 24 hours (hr.), andthe reaction time can be extended indefinitely according torequirements.

Some embodiments and test results of the above method embodiments of thepresent disclosure are further illustrated and described below, but thatare not limited as described here.

To improve the intrinsic conductivity and reduce the charge transportbarrier, achieving multicomponent metal oxide coatings with mixedvalence and homogeneous distribution is a highly challenging, buteffective strategy to enhance the electron hopping process and thusconductivities. Ultrathin, highly continuous deposition of amorphousmulticomponent metal oxides is therefore an optimal and desirable modelfor OER electrocatalysts. As KMnO₄ is a strong stain reagent on varioussurfaces (e.g. fabrics, plastic, and even human skin), it is inspired toutilize this nature of KMnO₄ to achieve strong film adhesion onarbitrary substrates without pyrolysis treatment. Co(OAc)₂ and KMnO₄interactions result in self-limited redox-coupled film growth governedby ligand coordination effects. For electrocatalytic OER applications,amorphous CMOH exhibits superior activities and durability to itscrystalline counterpart and also benchmark RuO₂. Examples of theexperimental part are presented as follows.

Preparation of CMOH Thin Films:

The reaction mixtures for deposition were prepared by dissolving cobaltprecursors (i.e. Co(OAc)₂, CoSO₄, and Co(NO₃)₂) and KMnO₄ in deionized(DI) water (18.2 MΩcm) with a typical Co/Mn mole ratio of 3/1. As asubstrate, we mainly used fluorine-doped tin oxide (FTO) glass obtainedfrom Hartford Glass. FTO was rinsed with acetone, isopropyl alcohol(IPA), DI water, and 5.2 M HNO₃ under sonication for 10 minutes,followed by the exposure to O₂ plasma (25 W) for 20 seconds to completethe cleaning process. The deposition area is typically 0.5×0.5 cm²,patterned by nail-polish oil masking. It also performed deposition oncopper foil, Ni foam, carbon cloth, glassy carbon electrode (GCE),SiO₂/Si wafers, and glass. In a typical deposition, substrates werevertically placed in reaction mixtures of KMnO₄ and Co(OAc)₂ with 500resale price maintenance (rpm) stirring at 80° C. for 15 minutes. Thesubscript of CMOH represents the anions of cobalt precursors used in thedeposition. CMOH without specific subscript refers toCo(OAc)₂-deposition. After the deposition, the coatings were rinsed withDI water and the nail-polish mask was removed with acetone. The CMOHannealing was carried out at 500° C. for 1 hour under argon to obtaincobalt manganese oxide (CMO) films. The temperature-dependent CMOHdeposition was carried out at room temperature, 50° C., 80° C., and 95°C.

Reaction mixtures with varied Co/Mn mole ratios were prepared(Co:Mn=1:3, 1:1, 3:1, 5:1, 7:1, and 9:1). For the redox deposition ofiron manganese oxide coatings, Fe(OAc)₂ (Acros Organics) is used as theprecursor with a Fe/Mn mole ratio of 3/1 in the reaction mixture. In thesynthesis of ternary metal oxide films, Co(OAc)₂, Fe(OAc)₂, and KMnO₄were mixed with a Fe/Co/Mn ratio of 1/2/1.

Electrochemical Measurements:

Electrochemical results were acquired using a three-electrode system ona CHI 614D Electrochemical Analyzer. FTO glass with CMOH coatings wasused as the working electrode, where a Pt plate and Hg/HgO were used asthe counter and reference electrodes, respectively. OER activities wereevaluated by linear sweep voltammetry (LSV) with a scan rate of 5 mV s⁻¹under 0.1 M KOH. All the overpotentials (η) were recorded at 10 mA cm⁻².The potentials presented herein are based on the reversible hydrogenelectrode (RHE) following the equation:

ERHE=E_(Hg/HgO)+0.098+0 0.059×pH  (1)

Faradaic efficiency (FE) was obtained using a gas chromatograph (GC)equipped with a thermal conductivity detector (TCD) to analyze thequantity of molecular oxygen. The FE was acquired from the ratio of O₂measured/O₂ theoretical, where O₂ theoretical was integrated from thecurrent-time (i-t) curve. A quartz crystal microbalance (QCM/CHI 401)was used to monitor the in situ growth of CMOH coatings at roomtemperature. The fundamental resonant frequency of QCM is 8 MHz. Theweight change was calculated using the Sauerbrey equation:

Δf=−(2×f ₀ ² ×Δm)/[A(ρ_(a) ×G _(a))^(1/2)]  (2)

where f₀ is the fundamental resonant frequency of QCM, ρ_(a) is thedensity of quartz (2.648 g cm⁻³), G_(a) is the shear wave velocity ofthe quartz crystal (2.947×10¹¹ g cm⁻¹ s⁻²), and A is the activeelectrode area of QCM. For all the QCM measurements, Au/quartzsubstrates were first kept in DI water until frequency equilibrium isreached. Afterwards, Co and Mn precursors were carefully injected intothe system to initiate coating growth. Pure Co(OAc)₂ and KMnO₄ were alsotested in QCM as control samples. To study the effect of counter ions,cobalt precursors with different anions of Co(OAc)₂, CoSO₄, and Co(NO₃)₂were used following the same deposition conditions. Sodium acetate(Acros Organics) was used as the source of the acetate anion.

Characterization:

Scanning electron microscopy (SEM) images were obtained using a FEIInspect F50 and Zeiss Supra 55 Gemini with acceleration voltages of10-20 kV. The X-ray photoelectron spectroscopy (XPS) measurements weredone on a PHI 5000 VersaProbe. The film composition profile was studiedby Arsputtering XPS with a removal rate of 3 nm min⁻¹. The grazingincident X-ray diffraction (GIXRD) was used to characterize CMOH thincoating with 1 degree (°) grazing angle on a Bruker D8 Advancediffractometer with a CuKα X-ray source. Field emission transmissionelectron microscopy (FE-TEM) images were collected with a FEI E.O TecnaiF20 G2 at 120 kV. TEM foils were prepared using a focused ion beam (FIB)using a SMI 3050. The CMOH/FTO samples were first coated with platinumand a subsequent carbon layer, followed by ion beam cutting andthinning. Samples were analyzed by energy dispersive X-ray spectroscopy(EDXS) under SEM and TEM. The Raman spectra were obtained using a WITecConfocal Raman Microscope with a 532 nm wavelength laser source. TheCMOH samples were deposited on gold substrates to enhance the Ramansignals via surface-enhanced Raman scattering. The X-ray absorptionspectra (XAS) were collected at 17Cl in the National SynchrotronRadiation Research Center, Taiwan (NSRRC) with transmission mode. Theroughness of CMOH films was analyzed by atomic force microscopy (AFM,Bruker Dimension Edge) with contact mode. The conductivity measurementwas conducted using a four-point probe on a Quatek 5601Y SheetResistivity Meter. The UV-vis spectra were obtained with a Jasco V-630UVvisible spectrometer. Inductively coupled plasma mass spectrometry(ICP-MS) measurements were carried out with a PerkinElmer ELAN 6100 DRCPlus for elemental analysis. To determine Co/Mn ratios, CMOH sampleswere dissolved in a solution composed of HNO₃ (60%) and H₂O₂ (35%) witha 2:1 volume ratio. To study the elemental leaching issue, the OERelectrolyte solution (0.1 M KOH) after 10 000 cycle sweeps was sampledto determine the contents of Co and Mn.

Simulation of CMOH Deposition Behavior:

Molecular dynamics (MD) simulations were carried out to investigate thegrowth of the CMOH □lm on the FTO surface. The cases of Co(OAc)₂ andCoSO₄ deposition were investigated. The composition of the MD cell inthe acetate system includes 1500 Co²⁺, 3000 OAc⁻, 500 MnO₄ ⁻, 500 K⁺,and 2000 H₂O (solvent), while that of the sulfate system includes 1500Co²⁺, 1500 SO₄ ²⁺, 500 MnO₄ ⁻, 500 K⁺, and 2000 H₂O. The crystalline tinoxide (SnO₂, 100×100×8 Å³) substrate was established to imitate FTOglass for the deposition. All simulations were computed by usingMaterial Studio software. COMPASS force field and NVT ensemble wereadapted for the simulations. The density of the liquid phase in eachsystem was set to be 1.0 g cm⁻³. The initial temperature of MDsimulations was 298 K until a thermal equilibrium was reached; then thetemperature was further increased to 353 K. This temperature settingcorresponds to the real reaction temperature. The pair distances betweenCo²⁺ and Mn⁷⁺ (in MnO₄ ⁻) to O on the SnO₂ surface, as well as Co²⁺ to Oin MnO₄ ⁻ (i.e. (MnO₄)—Co complexes), were analyzed. The metalcation-to-O distances shorter than 3.0 Å were recognized to be due tothe bond formation for yielding precipitate. This linking process wasrepeated five times for every 75 picoseconds. The following examplesillustrate results and discussion.

Deposition and Characterization of CMOH Coating:

The solution processed deposition of binary CMOH films was carried outin a single-step redox process under ambient conditions. The aqueousreaction mixtures were prepared by dissolving various Co_((II))precursors with KMnO₄ (as the metal-containing oxidant) without anyadditives (e.g. organic solvents, surfactants, polymers, etc.). Toclearly demonstrate film deposition, transparent FTO was selected as thesubstrate as shown in FIGS. 2-10. The one-step CMOH deposition producedby the Co(OAc)₂ precursor (i.e. CMOH_(acetate)) can be accomplished bydipping pristine FTO in the reaction mixture, aging for a certain periodof time, and removing it after complete deposition (FIGS. 2-4). Neitheran inert atmosphere nor delicate operation was required. The uniformlydark contrast of deposition can be obtained with the homogeneousdistribution of cobalt and manganese as proven by EDXS (FIGS. 44-47).The ICPMS analysis confirms the bulky composition of Co/Mn=2.92 (Table1), similar to the selected-area composition of 3.08 acquired by EDXS.

TABLE 1 The elemental composition of CMOH films with varied depositiontimes and different precursor Co-to-Mn ratios Deposition time 1 min 5min 15 min 30 min 60 min Co/Mn 3.03 2.87 2.92 2.86 2.91 ratios of CMOHCo(II)-to-Mn(VII) precursor ratios 3:1 5:1 7:1 9:1 Co/Mn 2.92 2.95 4.465.72 ratios of CMOH

Compared to other solution-based depositions, homogeneous binaryelemental distribution generally requires specific reaction conditionsdue to potential mismatch in properties (e.g. hydrolysis rates, K_(sp)constants, thermal stabilities, etc.) between precursors. The fixedelectron exchange stoichiometry dependent on the redox synthesisprovides a reliable composition homogeneity for multi-precursordeposition. Different from typical dip-coating or polymer-assisteddeposition, our procedure does not need thermal annealing to eliminateorganic/polymer components and to consolidate coating adhesion, thuspreserving the amorphous feature.

The SEM image (FIG. 5) of the CMOH film deposited on a SiO₂ wafer showsthe smooth and highly continuous coating with a root-mean-square (RMS)value of 3.16 nm as acquired using AFM (FIG. 6). The GIXRD patterns ofthe samples deposited using Co(OAc)₂ and CoSO₄ precursors(CMOH_(sulfate)) show no diffraction peaks, indicating the amorphousfeatures of CMOH coatings (FIG. 7). The Raman spectra of CMOH_(acetate)exhibit a broad band at 599 cm⁻¹ (FIG. 48), which is also in agreementwith the presence of amorphous cobalt oxide. The signals of amorphousmanganese oxide are difficult to recognize due to their relatively smallamounts (<25%), significant peak broadening, and similar Ramanwavenumbers to cobalt oxide. By annealing CMOHsulfate at 500 C for onehour, the coatings are shown to be crystalline corresponding to thespinel Co₃O₄ phase, denoted as cobalt manganese oxide (CMO_(sulfate))(FIG. 49). To further verify the film crystallinity, the focused ionbeam (FIB)-cut foils of CMOH_(acetate) were characterized byhigh-resolution TEM. The TEM image of uncalcined CMOH_(acetate) clearlyshows amorphous features without any ordered lattice fringes (FIGS. 8and 50), in agreement with the amorphous deposition shown in FIG. 7. Thecoating cross-section is highly continuous and pinhole/void-free with amajor thickness of 6-10 nm, comparable with the thickness of 11 nm underAFM (FIG. 51). At this thickness, the reported coatings fabricated byphysical deposition still remain discontinuous. The annealedCMO_(acetate) films exhibit crystalline lattices with a d-spacing of0.244 nm, corresponding to the (311) plane of spinel Co₃O₄ (FIG. 9). Thefilm conductivities are summarized in Table 2, showing that CMOHcoatings exhibit the sheet resistance in the range of 7.4×107 to13.0×107 (⋅□⁻¹). The annealed CMO generally displays a smaller sheetresistance (as low as 0.469×107 ⋅□⁻¹) than amorphous CMOH.CMOH_(sulfate) is slightly more conductive than CMOH_(acetate).Different Co/Mn ratios show insignificant influence on the filmresistance. The controlled samples of manganese oxide coatings possess asheet resistance that exceeds measurement limits, suggesting thathomogeneous binary oxide coatings exhibit a much lower sheet resistancethan single oxides.

TABLE 2 The conductivity of CMOH and CMO coatings with varied divalentcobalt precursors and Co-to-Mn ratios Sheet resistance(Ω□⁻¹) × 10⁷Co(OAc)₂-precursor coating Co:Mn = 3:1 13.0 Co:Mn = 7:1 11.5 Co:Mn =3:1(annealed) 10.6 Co:Mn = 7:1(annealed) 8.95 CoSO₄-precursor coatingCo:Mn = 3:1 7.41 Co:Mn = 7:1 7.40 Co:Mn = 3:1(annealed) 0.470 Co:Mn =7:1(annealed) 0.649

Large Scale Fabrication and Properties of CMOH:

With the easy operation procedure, we attempted to achieve highthroughput fabrication by parallel dipping of numerous substrates in onebatch of the reaction mixture. FIGS. 11 and 12 show the success ofparallel deposition to produce six individual, uniform and well-definedCMOH_(acetate) coatings on FTO, more efficient than batch-to-batchdeposition such as spin coating. In addition, shape- and size-specificdeposition can be controlled by masking techniques. FIG. 13 shows therealization of large-scale arrays of square-shaped deposition over 10×10cm² defined by the resin-based masks. All the patterned units of CMOHare well defined in shapes, interval distance, and show highly similarcontrast. The scalable and throughput redox deposition appears highlypractical for massive production.

Notably, the as-coated CMOH also exhibits high visible-lighttransparency. By changing the deposition temperatures (FIG. 14), thecoating contrasts become darker as the temperature increases indicatingthe formation of a thicker coating. The transparency (at 550 nm) of CMOHdeposited at room temperature, 50° C., 80° C., and 95° C. was measuredto be 99.2%, 98.4%, 97.4%, and 95.2%, respectively (FIG. 15).Room-temperature CMOH forms a thin, uniform, and barely visible coating(see the arrows in FIG. 14). This property becomes relevant as binarymetal oxides recently gained attention as materials for transparentOER-active thin films. Moreover, CMOH exhibits a higher transparencythan the reported binary FeNiO_(x) that is active forphotoelectro-chemical cells (PEC) and sunlight-driven water splittingapplications.

Film adhesion is a crucial concern particularly for low-temperaturedeposition. As shown in FIG. 16, we conducted peel-off tests withScotch-tape on CMOH over 100 cycles. There were no appreciable filmdetachment or breaking observed. This strong adhesion is comparable tothe annealed crystalline CMO samples, which allows CMOH to be directlyused in the amorphous form. The step coverage studies show that CMOHcoating can be deposited along the top, side wall, and base of SiO₂trenches with a similar thickness of 9.1 nm, 9.5 nm, and 10.1 nm,respectively (FIG. 17); hence, homogeneous CMOH deposition stronglyattached on 3D complex architectures and porous tunnels can be expected.As shown in FIGS. 18a-18f , it is further successfully demonstrated CMOHdeposition on complex 3D structures (Ni foam) and cylinder-shapesubstances (screw pairs). On different types of surfaces, includingrigid glass, soft plastics (polyethylene terephthalate, PET), wood, andmetal foils (Cu), CMOH deposition exhibits strong adhesion and highhomogeneity (FIGS. 18a-18f ). In particular, the coating on PET cancompletely tolerate the bending strain without appreciable film breakingafter 100 cycle tests under 180 folding, agreeing with the excellentadhesion and mechanical properties.

Redox Interaction:

To verify the underlying principles of CMOH formation, oxidation statesof cobalt and manganese are investigated. In the XPS spectra (FIG. 19),the binding energy of Co 2p_(1/2) at 796.2 eV, 2p_(3/2) at 781.1 eV, andthe satellite peaks at 790.8 eV reveals the presence of Co(III). The XPSdata also show that signals from Mn 2p_(3/2) and 2p_(1/2) are 641.8 and654.2 eV, respectively, where Mn³⁺ and Mn⁴⁺ are barely distinguishable(FIG. 20). In the spectra of O 1s, the strong hydroxide signals can beobserved at 532.0 eV and a weak signal of O²⁻ at 530.0 eV (FIG. 21),showing similar ratios to metal oxyhydroxide species. After filmannealing at 500° C., the hydroxide signals significantly decrease whileO²⁻ signals are much stronger due to the conversion of amorphousoxyhydroxide to crystalline oxide (FIGS. 52-54). Therefore, the presenceof Co_((III)) and hydroxide/oxide signals indicates CoOOH-like amorphousCMOH. It is further conducted X-ray near edge structure studies of CMOHconfirming the oxidation states of Co and Mn. The K-edge signals of Co(FIG. 22) are observed at 7728.6 eV, corresponding to the reportedoctahedral-coordinated Co_((III)) at 7729 eV, which further agrees withthe XPS data. The Mn K-edge spectra (FIG. 23) show the peak at 6563 eV,nearly identical to that of MnO₂ at 6562.2 eV instead of Mn_((III)) at6554.2 eV. The results, therefore, unambiguously confirm the presence ofCo³⁺ and Mn⁴⁺ in CMOH coatings. As the elemental analysis highly agreeswith ideal redox stoichiometry between Co²⁺ and Mn⁷⁺ as 3:1 (i.e. 2.86to 3.03, see Table 1), it is evident that the redox-driven CMOHdeposition can be expressed as Co_(1−x)Mn_(3x/4)OOH. Therefore the netredox equation is shown as following:

9Co²⁺ _((aq))+3MnO₄ ⁻ _((aq))14H₂O_((I))→Co₉Mn₃O₂₆H_(13(s))+15H⁺_((aq))  (3)

The film composition profiles of CMOH acquired by XPS (FIG. 24) exhibituniform Co:Mn mole ratios (i.e. 3.30 by average) from top to bottom,corresponding well with the redox-coupled film growth as shown inequation (3). The depth profile studies show residue signals for sulfurand carbon to be less than 0.01%.

Coating Formation Process:

QCM is conducted to monitor the loading mass and film growth of CMOH onAu/quartz substrates in situ. 11,56 First, it is conducted controlexperiments of deposition with the precursor either Co(OAc)₂ or KMnO₄only. The profiles of FIG. 25 show no increase of mass loading in theCo(OAc)₂-only case, while the appreciable film formation can be observedin the KMnO₄-only case. In addition, the deposition tests oncarbon-based and transparent substrates also show that onlyKMnO₄-deposition contributes to film formation (see FIGS. 55-56). Thisfurther confirms that the strong oxidative staining ability of KMnO₄ iscritical for film formation and immobilization on substrates. For thetests combining Co(OAc)₂ and KMnO₄, the QCM profiles indicate a muchfaster and greater increase in mass loading than the previous twocontrol experiments, verifying the redox-coupled nucleation.

As Co²⁺ is the quantity-dominant species in the reaction mixture,attraction between the substrate-anchored MnO₄ anion and Co²⁺ cationcould facilitate on-site redox interaction on the substrate surface toform CMOH coating. Despite the reported studies of cobalt oxyhydroxidepreparation via the redox route (e.g. interaction of Co²⁺ and S₂O₈ ²⁻ toyield CoOOH), their thin film deposition has been rarely recognizedSuccessful cobalt incorporation into the thin film form was firstrevealed in this work through redox interaction with KMnO₄.Theoretically, each Mn⁷⁺ would transfer charge directly to threeneighboring Co²⁺ ions, giving the probability to constructinterconnected networks holding multiple Co atoms with one Mn togetherthrough oxygen-bridged bonding. As a result, this network-likenucleation may favor the formation of continuous coating even at theultrathin scale of several nanometers, rather than island-like,discontinuous deposition frequently observed in physical vapordeposition. Therefore, KMnO₄ is proposed to play the dual roles of botha surface-anchoring oxidant and a cobalt-fixation reagent in the binaryoxide deposition process.

Effect of the Precursor Anion on Deposition:

To investigate the control parameters of the film thickness, notably, itis observed that film growth was highly dependent on the counterions ofcobalt precursors. Under identical conditions, as shown in FIG. 26,CoSO₄ and Co(NO₃)₂ precursors exhibit the general trend that the CMOHthickness is proportional to the deposition time. Their deposition rates(0.059 μg min.⁻¹ for CoSO₄ and 0.086 mg mini for Co(NO₃)₂) are 6 to 9times faster than that for Co(OAc)₂ (0.0097 μg min.⁻¹). In fact, theCoSO₄ and Co(NO₃)₂ deposition can continuously go beyond several hoursto generate a much thicker coating. On the other hand, theCo(OAc)₂-deposition grew linearly at the first 50 minutes, and thenbecame saturated at 60 minutes with the maximized loading mass of 2.97μg cm⁻² (FIG. 26). This self-limiting phenomenon was also observed atdifferent deposition temperatures of CMOH_(acetate). The results clearlyshow that the deposition thicknesses produced vary depending on theprecursor anions. As shown in FIG. 28, the cross-sectional SEM image ofCMOH_(sulfate) for 2 hours deposition, featured with the homogeneouselemental distribution for 180 nm thickness with the absence of cracksor pinholes/voids. Together with the ultrathin thickness ofCMOH_(acetate), the film thickness can be controlled ranging fromseveral nanometers up to submicrons via precursor anions. To lower theinterfacial barrier of electrocatalysis, Co(OAc)₂ deposition is adoptedto produce ultrathin CMOHacetate for the later OER studies.

To further investigate the effect of anions, it is carried out thecontrol experiments by adding acetate ions to CoSO₄-deposition (FIG.27). The addition of two acetate ion equivalents, whose quantity iscomparable to that of Co(OAc)₂, yielded a nearly identical saturationtime and coating mass to that of Co(OAc)₂— deposition. However, theaddition of one acetate ion equivalent, which corresponds to half ofthat in Co(OAc)₂, behaved similar to that of CoSO₄-deposition but nosaturation was observed. In addition, the loading mass lies in betweenthose of CoSO₄ and Co(OAc)₂ deposition. These phenomena clearly indicatethat the self-limiting film growth is due to the presence of the acetateanion. It is also observed similar film growth inhibition with theidentical anion combination of Fe₂(SO₄)₃ and Ni(OAc)₂ to this case.Indeed, acetate anions may act as a buffer species to influence the pHconditions and also the deposition. Their pH variation studies werepartially effective but comprehensive understanding of the saturationgrowth of binary iron nickel oxides remained uncertain. Since theacetate anion possesses the stronger intrinsic coordination capabilitythan the other two, it is most likely that the ligand coordinationeffect may rationalize the anion influence.

To verify the coordination effect, the hexadentate ligand ofethylenediaminetetraacetic acid (EDTA) is added as the much strongercoordination ligand than acetate for comparison. No coating formationcan be observed in the presence of EDTA (FIG. 27), indicating that itsability to coordinate with Co²⁺ significantly influences CMOHdeposition. It is therefore compared the oxidation potentials of EDTA-and acetate-coordinated Co_((II)) under the deposition conditions. TheCo²⁺/Co³⁺ oxidation potentials in the presence of acetate and sulfateanions are 1.55 and 1.63 V, respectively (see (I) and (V) in FIG. 29).With the addition of the acetate ligand to Co sulfate (mole ratio 2 to1, see (II) and (III) in FIG. 29), the Co²⁺/Co³⁺ oxidation potentialsdecrease which is eventually similar to that of Co(OAc)₂. On the otherhand, the addition of a stoichiometric amount of sodium sulfate toCo(OAc)₂ shows no significant change of Co²⁺ oxidation peaks. By addingEDTA to CoSO₄, a drastic decrease in the Co²⁺/Co³⁺ oxidation potentialto 0.67 V has been observed (see (IV) in FIG. 29). This oxidationpotential drop for Co²⁺ should make the redox deposition even easier andmore spontaneous. But the observed absence of CMOH indicates that theligand effect governs the deposition growth rather than the redoxpotential changes. The EDTA ligand traps Co²⁺ tightly and may keep thecoordinated Co²⁺ away from the substrate for film growth. Compared toEDTA, the relatively labile and weak coordination of acetate may resultin the suppressed deposition rather than a complete stop. It is alsolikely that the addition of coordination ligands may establish a newequilibrium unfavorable for the CMOH deposition. Thus, the precursoranion effect could be mainly attributed to the ligand coordinationability. As a result, the precise control of the thickness can befeasible through the proper selection of ligand additives.

Simulation Study of CMOH Growth:

FIGS. 30-33 shows the MD simulation studies of (MnO₄)—Co complexes thatare yielded after the redox reaction in sulfate (see (a) in FIG. 30) andin acetate (see (b) in FIG. 30) deposition. The different gray-leveldots represent O, Co, and Mn, respectively. Two different (MnO₄)—Cocomplexes, one forms in solutions as a colloidal precipitate (see (c) inFIG. 30) while the other binds with SnO₂ substrates (see (d) in FIG. 30)as film deposition, were depicted and studied. The quantity of colloidalcomplexes in sulfate solution was observed to be larger than that inacetate solution.

By correlating the numbers of (MnO₄)—Co colloidal complexes formedversus simulation time (FIG. 31), it is found that the formation ratesin the sulfate solution are more than those in the acetate solution. Thesaturated number of (MnO₄)—Co colloidal complexes in acetate solution isfound to be 480. Since the total number of Co ions is 1500 at theinitial stage, about one-third of Co ions in the acetate solution formthe colloidal complexes. For ions in sulfate solution, the maximumnumber of colloidal complexes at the end of our simulation time is 780,that is, larger than one half of the number of Co ions.

Co ions are not only bonded to the oxygen in MnO₄, but also to theoxygen of the SnO₂ surface to form deposition (see (e) in FIG. 30(sulfate system) and (f) in FIG. 30 (acetate system)). Similarly, thenumber of surface-linked complexes in sulfate solution was larger thanthat in acetate solution. Within the simulation time period, the numberof surface-linked complexes in the acetate solution saturates but not inthe sulfate case (FIG. 32). These simulation results agree well with theaforementioned experimental observation of ligand-governed CMOHdeposition.

To further investigate the formation of the (MnO₄)—Co complex, wecalculated the radial distribution function (RDF) of Co ions to O insulfate (g_(Co—O(sulfate))) and O in acetate (g_(Co—O(acetate))). InFIG. 33, g_(Co—O(acetate)) has a peak appearing around 3.5 Å, whileg_(Co—O(sulfate)) has no corresponding peak. This means that acetateanions closely surround Co ions with a high local density rather thanbeing homogeneously dispersed in the whole solution cell. Suchacetate-ion aggregation restricts the coordination of Co²⁺ to otheroxygen atoms (e.g. O of SnO₂ substrates), and may also accumulate agreat negative charge barrier to repulse away the MnO₄ ⁻ anion fromfacilitating redox interaction. The significantly inhibited CMOHdeposition can also be explained to be due to change in ligands, fromacetate to EDTA.

Electrocatalysis of the OER:

FIG. 34 shows a comparison of a linear sweep voltammogram (LSV) ofamorphous CMOH_(acetate) and crystalline CMO_(acetate) deposited on FTOunder 0.1 M KOH. No appreciable OER activities can be observed with bareFTO up to 1.9 V. The OER onset and over potentials (η at 10 mA cm⁻²) foramorphous CMOH are at 1.28 V and 390 mV, respectively. Crystalline CMOshows a higher onset potential than CMOH at 1.47 V with η of 460 mV. Atan over potential of 400 mV, the current density of CMOH is 11.60 mAcm⁻², which is 4.7 times greater than that of CMO. As compared tobenchmark RuO₂, CMOH has the smaller onset potential of 180 mV and anoverpotential of 200 mV. In FIG. 35, CMOH exhibits the favorable OERkinetics with a smaller Tafel slope of 60.9 mV dec⁻¹ than that of CMO(72.8 mV dec⁻¹). The faradaic efficiency of CMOH was measured to benearly 100%, indicating that no side electrochemical reaction occurredduring the OER (FIG. 57). In general, the greater conductivities enablethe higher electrocatalytic performance. Amorphous CMOH, despite itslower conductivity, exhibits higher OER performance as compared tocrystalline CMO (Table 2), clearly showing the intrinsic OER superiorityof amorphous materials over crystalline ones. The comparison of OERperformance with the reported thin coating is summarized in Table 4.

Metal oxyhydroxide (e.g. CoOOH, NiOOH) has been identified as theactivity species for the OER. Thin amorphous metal oxyhydroxides arecommonly obtained from the electrochemical conversion of metalhydroxides as pre-catalysts during the OER, rather than produced bydirect deposition. Electrochemical conditioning is needed to transformcrystalline metal hydroxides to oxyhydroxides for enhanced OER activity.It is found that no appreciable electrochemical conditioning was neededfor CMOH to enhance OER performance (FIG. 34) since it may be already inthe form of oxyhydroxide.

To investigate the optimal composition, the coatings with varied Co/Mnratios have been produced by changing the precursor ratios in theinitial reaction mixtures (See Table 1). By increasing the contents ofcobalt precursors, the coatings are generally produced with greaterCo/Mn ratios. Due to the redox interaction, the Co/Mn ratios of coatingsare shown to be less varied (i.e. 2.92-5.72) compared to those of thereaction mixtures ranging from 3/1 to 9/1. The Co/Mn precursor ratio of7/1 yielded the most active coatings (CMOH_(7/1)) with the smallestonset potential among the others (FIGS. 36, 58 and Table 3). The cobaltXPS data of CMOH_(7/1) (FIG. 59) show a high similarity to those ofCMOH3/1, indicating that Co³⁺ is still the main species of the coatings,instead of Co²⁺. This might also suggest that Co3+ is the active speciesresponsible for the OER rather than Mn sites. Compared to the idealredox stoichiometry and binary compositions, the relatively lowerquantity of Mn in CMOH_(7/1) may suggest Mn⁴⁺ substitution by Co³⁺yielding cation vacancies in the framework of films due to chargecompensation. These defects in small quantity could increase materialconductivity and improve catalytic activities as those observed inCMOH_(7/1) (see Table 2), but large contents of defects can weaken thestructure stability. The significant drop of activities at Co/Mn=9/1 isthus attributed to the observed incomplete film formation due to evenhigher cation vacancies that collapse the framework of CMOH.

Table 3 OER Performance

TABLE 3 OER performance Electrochemical performance Onset Potentialpotential (V)@ Tafel slope Sample (V) 10 mA cm⁻² (mV dec⁻¹) Co:Mn = 1:31.50 1.73 64.0 Co:Mn = 1:1 1.47 1.71 62.9 Co:Mn = 3:1 1.43 1.68 62.0Co:Mn = 5:1 1.41 1.67 61.2 Co:Mn = 7:1 1.28 1.62 60.9 Co:Mn = 9:1 1.451.77 77.8 CMO 1.47 1.69 72.8 RuO₂ 1.42 1.82 73.6 CMOH/Nickel foam 1.231.54 80.1 CMOH/Cu foil 1.25 1.64 78.1 CMOH/Carbon cloth 1.36 1.57 68.9CMOH/GCE 1.44 1.65 63.3

TABLE 4 OER performance comparison between CMOHacetate and the reportedelectrocatalysts Onset Potential potential @10 Electro- Sample (V) mAcm⁻² lyte Reference CoPi 1.64 N/A 0.1M KPi 1 Ni_(0.9)Fe_(0.1)O_(x) 1.531.57 1M KOH 2 Mn₂O₃ N/A 1.81 0.1M KOH 3 FeCoNiO_(x) 1.42 N/A 0.1M KOH 4Fe₄₀Co₄₀Ni₂₀O_(x) 1.42 N/A 0.1M KOH 5 Mn₂O₃ 1.40  1.67^(a) 1M KOH 6Mn/Co—CB 1.45 N/A 0.1M KOH 7 CoO_(x)—CoSe 1.50 N/A 0.1M KOH 8 Cu₃P 1.60N/A 0.1M KOH 9 CMOH 1.28 1.62 0.1M KOH The present disclosure

The OER stability tests of i-t curves (FIG. 37) were conducted at acurrent density of 10 mA cm⁻² for 60,000 seconds continuously. Noappreciable drops (<2%) of current density for CMOH can be observed,while CMO shows the decrease of 18% of the current density. RuO₂displays the more severe current density decrease of 67%, much largerthan those of both CMOH and CMO. In addition, we compared cyclic LSVtests for 10,000 scans. Amorphous CMOH exhibits the nearly identicalcurves to the first run (FIG. 38), while crystalline CMO shows thecurrent density decay of 16% with the increase of overpotential by 16mV. Benchmark RuO₂ exhibits even greater current density decay of 18%and an increase of overpotential by 44 mV after the tests. In fact, bothCMOH and CMO can be more stable than RuO₂ over the whole cycle tests. Inthe TEM characterization, the CMOH after the 10,000 cycle tests stillremains amorphous without any crystalline features (FIG. 10), indicatingthat the exceptional oxygen evolution activities and stability were dueto the amorphous characteristics despite the possible local structuralrearrangement during the OER.

It is further conducted leaching studies by sampling the OER electrolytesolutions (0.1 M KOH) after 10,000 cycles. The ICP-MS data show thatcrystalline CMO coatings release both Co and Mn twice more thanamorphous CMOH, which may explain the poor stability of CMO compared toCMOH over time (FIG. 37). The spinel phase has been recognized as anunfavorable structure to be transformed to oxyhydroxide.Well-crystalline spinel CMO with a rigid, ordered coordinationenvironment may restrict the flexibility of structural alteration. Thehigh lattice stress due to phase transformation increases theprobability of irreversible bond breaking, leading to the observed Coand Mn leach. In contrast, the disordered amorphous CMOH is morestructurally flexible to tolerate the greater degrees of structuralrearrangement, including Co³⁺/⁴⁺ exchange in OER mechanisms.

It has studied the thickness effect on the OER by varying depositiontime and with different precursors of Co(OAc)₂ and CoSO₄. As shown inFIG. 39, under the same deposition time, the Co(OAc)₂-coatings generallyexhibit better activities than CoSO₄— coatings. The nearly identical OERactivities between 15 minutes and 60 minutes Co(OAc)₂-coatings consistof the observed self-limiting growth and negligible difference inthickness (also see FIGS. 60-61). For the case of CoSO₄-coatings, thelonger deposition time results in the weaker OER performance closelyassociated with their drastic thickness difference. As ultrathin coatingfacilitates charge transport in electrocatalysis, the thickness controlof CMOH through the ligand coordination effect can be a promising routeto manipulate OER performance.

OER on Various Substrates:

With the substrate-universal deposition and easy operation, it is testedultrathin CMOH on various substrates commonly used for the OER,including metal foils (Cu foils), carbon cloth, 3D Ni foams, and aglassy carbon electrode (GCE). As shown in FIGS. 40-43, theelectrochemical results generally show the superior OER enhancementcompared to the uncoated substrates, suggesting (1) the stronginterfacial contacts between CMOH and the substrates forelectrocatalysis, and (2) the exceptional OER activity of amorphous CMOHcoatings. Notably, the LSV curve for bare Ni foam shows the oxidationpeak at 1.34 V, corresponding to the transformation ofNi_((II))/_((III)). However, the CMOH coating on Ni foam does notreflect the Ni_((II))/_((III)) signal, but it exhibits an OER onsetpotential of 1.234 V which is very close to the theoretical one (1.23 V)together with a small overpotential of 0.31 mV. The improved OERperformance has been summarized in Table 3. Under a 3 V electrolysissetup made by a series connection of two commercial batteries (1.5 V foreach), the operation video clips show vigorous O₂ bubbling solely fromthe CMOH coated area rather than the uncoated ones. Compared toCMOH-coated electrodes, the commercial pristine carbon rods and Pt wiresare relatively weaker in O₂ production under the same conditions. Theseresults confirm that CMOH is responsible for the oxygen evolution withthe superior OER activity to the substrates, including highly conductiveand electrocatalytically active copper foils and Ni foam.

Ternary Oxide Film Deposition:

Following the deposition principle above, it is explored diverse filmcompositions by replacing Co²⁺ with other transition metals, such asFe²⁺. The preliminary results show the success of iron manganese oxidecoatings with a Fe/Mn ratio of 2.39, suggesting the feasibility ofvarious metal oxide combinations through the redox protocol.Furthermore, with the presence of both Co²⁺ and Fe²⁺ with KMnO₄, theternary iron-cobalt-manganese oxide coatings on FTO have beensuccessfully produced, in which their component ratios are similar tothe precursor ratios (Fe:Co:Mn=1:2.11:0.77, see FIGS. 62-65). The metalion fixation role of KMnO₄ has been observed again for both Co²⁺ andFe²⁺. According to previous work, metal-containing oxidant KMnO₄ in theredox synthesis can potentially be replaced by other oxometallates (i.e.K₂Cr₂O₇), not limited to KMnO₄ only. The realization of diversecombinations of multi-component amorphous coatings can be thussystematically studied via this redox protocol.

In summary, the scalable, solution-processable protocols formulticomponent ultrathin metal oxide coatings capable of achievingpinhole-free, continuous, and substrate universal deposition. Theredox-coupled film formation was proved critical for film growth,fixation, and homogeneous elemental distribution. As there is no moreneed for pyrolysis treatment, this protocol is a suitable alternativefor amorphous deposition and substrates with low thermal durability.CMOH thickness and compositions can be controlled by means of ligandselection. This protocol might be useful for the fabrication of wearablesemiconductor devices, such as gate material deposition. For oxygenevolution, the new exploration of multicomponent amorphous metal oxides(e.g. more than four different metals) can be pursued for even greaterdurability and efficiency. The high transparency and film integrity bythe redox protocol may open a new avenue for light-assisted PECapplications.

In addition, in an embodiment of the present disclosure, the methodfurther includes a step (S3) after the step (S2), wherein the step (S3)includes: causing the metal oxide hydroxide film to be calcined by acalcination process in a calcination temperature range and under a gasenvironment to produce a calcined metal oxide film, wherein acalcination temperature range can be from a phase transition temperatureto a physical limit temperature of the material, such as the calcinationtemperature ranges from 250 to 800 degrees Celsius (for example, samplesin FIGS. 66a, 66b, and 66c are implemented at 400, 600, and 800 degreesCelsius, respectively). In addition, the gas in the gas environment maybe air in an atmospheric environment, argon (Ar), nitrogen (N), oroxygen (O₂), and a duration of the calcination process may be rangedfrom 1 to 12 hours.

In an embodiment of the present disclosure, the substrate may beselected from a group consisting of following object, such as siliconcrystal board, organic polymer (for example, a plastic plate shown inFIG. 67a or rubber balloons shown in FIG. 67b , etc.), organic matter(for example, a wood shown in FIG. 68a , a plastic plate shown in FIG.68b , rubber balloons shown in FIG. 68c , etc.), carbon material (forexample, a graphene shown in FIG. 69a , a carbon cloth shown in FIG. 69b, a carbon/plastic composited substrate shown in FIG. 69c , etc.),carbohydrate (for example, biocellulose such as a wood shown in FIG. 70or artificial leather, but that is not limited as described here),glass, nickel foam, metal, metal oxide, and glassy carbon electrode, butthat is not limited as described here. For example, the glass mayfurther be selected from a Fluorine-doped Tin Oxide (FTO) conductiveglass plate or an Indium Tin Oxide (ITO) conductive glass plate, butthat is not limited as described here.

Notably, the above embodiments of the present disclosure areparticularly related to an electroless deposition of a metal oxide filmon various substrates (especially a plastic organic substrate, such aspolyethylene terephthalate, polyurethane, polymethyl methacrylate,polyethylene naphthalate, or polycarbonate) in a liquid environment.

The method has at least the following advantages: the metal oxidehydroxide film and the metal oxide film are still continuous in anultra-thin state, only the thickness a few nanometers such as 5 nm isrequired to form a film.

In addition, the metal oxide hydroxide film and the metal oxide filmhave high activity and stability in an oxygen evolution reaction.

In addition, the metal oxide hydroxide film and the metal oxide filmhave strong adhesion to FTO conductive glass, ITO conductive glass,silicon wafer, wood, glass, nickel foam, plastic, metal substrate,carbon material, glass carbon electrode, and have low interfaceresistance to a conductive substrate.

In addition, the metal oxide hydroxide film can be uniformly coated on asubstrate with a complex structure because of the high permeability ofthe solution and can be coated on low environmental tolerance (such aslow pressure, high temperature, and insulator) substrates due to lowtemperature and room pressure reflection conditions.

In addition, the metal oxide hydroxide film is produced with adding nosurfactant, no vacuum environment, no valuable equipment, low cost, andlow pollution.

In addition, the metal oxide hydroxide film and the metal oxide filmhave characteristics, such as uniformly distributed elements (redoxelectronic measurement), flat surface, uniform thickness and excellentstepping coverage efficiency (solution permeability).

In addition, the metal oxide hydroxide film and the metal oxide filmhave good transparency and uniform appearance.

In addition, the metal oxide hydroxide film has flexible property.

In addition, the metal oxide hydroxide film and the metal oxide film canbe used for large-scale coating and pattern transfer reproduction.

In addition, the metal oxide hydroxide film and the metal oxide film canprecisely be controlled to a ratio between constituent metals.

In addition, the metal oxide hydroxide film has a structure belonging tothe amorphous form.

The present disclosure has been disclosed in its preferred embodiments,and it is not intended to limit the disclosure, and those skilled in theart can make various changes and modifications without departing fromthe spirit and scope of the disclosure. Therefore, the scope ofprotection of the present disclosure is subject to the definition of thescope of the appended claims.

What is claimed is:
 1. A method of depositing a metal oxide film in aliquid environment, comprising steps of: (S1) dissolving an oxidizingagent in a solvent with hydrogen bonds to form a solution; and (S2)placing a substrate into the solution for performing a depositionreaction to deposit a metal oxide hydroxide film on the substrate;wherein the oxidizing agent is potassium permanganate, potassiumchromate, or potassium dichromate, a reaction temperature of thedeposition reaction ranges from 1 to 99 degrees Celsius, and a reactionpressure environment of the deposition reaction is an atmosphericpressure environment.
 2. The method of depositing the metal oxide filmin the liquid environment as claimed in claim 1, wherein in the step(S1), further comprises a step of mixing a reducing agent and theoxidizing agent based on a molar ratio of the reducing agent to theoxidizing agent, in order to dissolve the oxidizing agent and theoxidizing agent in the solvent with hydrogen bonds to form the solution.3. The method of depositing the metal oxide film in the liquidenvironment as claimed in claim 2, wherein the reducing agent isselected from the group consisting of a divalent cobalt compound, adivalent iron compound, a divalent nickel compound, a divalent manganesecompound, and a first transition metal ionic compound.
 4. The method ofdepositing the metal oxide film in the liquid environment as claimed inclaim 2, wherein the molar ratio of the reducing agent to the oxidizingagent ranges from 9:1 to 1:3.
 5. The method of depositing the metaloxide film in the liquid environment as claimed in claim 1, wherein inthe step (S1), further comprises a step of adding an additive containingan anion into the solution, wherein the anion of the additive isselected from metal salt ions.
 6. The method of depositing the metaloxide film in the liquid environment as claimed in claim 1, furthercomprising a step (S3) after the step (S2), wherein the step (S3)comprises: causing the metal oxide hydroxide film to be calcined by acalcination process in a calcination temperature range and under a gasenvironment to produce a calcined metal oxide film, wherein thecalcination temperature ranges from 250 to 800 degrees Celsius.
 7. Themethod of depositing the metal oxide film in the liquid environment asclaimed in claim 6, wherein the gas in the gas environment is air in anatmospheric environment.
 8. The method of depositing the metal oxidefilm in the liquid environment as claimed in claim 6, wherein the gas inthe gas environment is argon, nitrogen, or oxygen.
 9. The method ofdepositing the metal oxide film in the liquid environment as claimed inclaim 6, wherein a duration of the calcination process ranges from 1 to12 hours.
 10. The method of depositing the metal oxide film in theliquid environment as claimed in claim 1, wherein the substrate isselected from a group consisting of silicon crystal, carbohydrate,glass, nickel foam, metal, metal oxide, organic matter, organic polymer,carbon material, and glassy carbon electrode.
 11. The method ofdepositing the metal oxide film in the liquid environment as claimed inclaim 1, wherein the solvent with hydrogen bonds is deionized water withan impedance of 18.2 MΩ·cm.